A flow is a diagram that can be used to show a variety of processes including task cases, business processes, and page flows. Frequently, they are used to provide a higher level, graphical view of tasks that can be accomplished using the pages in the design.
Flows are managed alongside pages in the Sitemap. To mark a page as a flow, right-click on the page in the sitemap and click Diagram Type -> Flow. This will change the icon in the sitemap in the designer as well as the prototype.
The wireframe pane is the drawing board for your flows. Flow widgets are available in the widgets pane to represent various steps in a process, and connectors are used to tie them together.